1. Field of the Invention
The invention is directed to a dispersion compensating optical waveguide fiber, a method of making the dispersion compensating optical waveguide fiber, and an optical communications link containing the dispersion compensating fiber and more particularly to a dispersion compensating optical waveguide fiber having a multiple core.
2. Technical Background
Optical telecommunication systems operating at very high bit rates typically require a low attenuation, large effective area optical waveguide fiber to achieve acceptable span lengths between electronic signal regenerator installations. The operating wavelength window extending from about 1450 nm to 1650 nm is attractive because of the low attenuation exhibited by silica based optical waveguide fibers over that wavelength range. To operate a system in this desired wavelength range, a transmission optical waveguide fiber was developed having a zero dispersion wavelength in or near this wavelength range. At the same time, the transmission optical waveguide fiber was designed to have large effective area in order to limit dispersion due to those non-linear effects that increasingly degrade the signal as signal power density increases.
A further advance in optical waveguide fiber design was made by locating the fiber zero dispersion wavelength outside the wavelength range over which the fiber was to be operated. By maintaining total dispersion magnitude greater than zero, preferably greater than about |0.5 ps/nm-km|, the negative impact of the non-linear phenomenon, four wave mixing, was essentially eliminated.
However, because the total dispersion of the improved fiber was not zero over the operating wavelength window and desired span lengths were long, there was a need to compensate for the total dispersion accumulated over a span length. The concept of a dispersion compensating fiber, having a total dispersion opposite in sign to that of the transmission fiber, was explored and appropriate dispersion compensating fibers were developed and proven successful. The dispersion compensating fibers developed typically incorporated a core refractive index profile having two or more distinct segments, a design that is generally more costly to manufacture in comparison to a step index profile or a graded index profile having only one segment.
An additional requirement was placed upon the dispersion compensating fiber in that high data rate telecommunication systems generally employ wavelength division multiplexing. If the dispersion compensating optical waveguide fiber was to be effective, compensation had to be relatively uniform over the band of wavelengths of the multiplexed signals. That is, the slope of the total dispersion of the compensating fiber had to be adjusted to achieve uniform compensation over an operating band of wavelengths.
Although, the segmented core dispersion compensating fibers have served to improve system performance, the total dispersion of the compensating fibers exhibit considerable curvature over the preferred operating wavelength range. Work has therefore continued to design dispersion compensating optical waveguide fibers that exhibit the desired total dispersion, linearity of total dispersion over the operating window, and afford relatively low manufacturing cost.
One aspect of the invention is a multiple core, dispersion compensating optical waveguide fiber that includes a center region surrounded by a clad layer. The center region includes at least two optical waveguide fiber cores. An optical waveguide fiber core is defined as the structure that serves to confine light within the fiber. Each of the cores has a refractive index profile. At least two cores have refractive index profiles that are different from each other. The refractive index profiles of the respective cores and their relative positioning within the center region provide for coupling of light from one core to at least one other core. The multiple core optical waveguide fiber is configured to have negative total dispersion and negative total dispersion slope over a pre-selected wavelength range.
In an embodiment of this aspect of the invention, the pre-selected wavelength range extends from about 1525 nm to 1565 nm, and the total dispersion slope is more negative that xe2x88x924.0 ps/nm2-km over the pre-selected wavelength range. In this embodiment, the ratio of total dispersion to the total dispersion slope can be approximately 50 nm at 1550 nm. Additionally, in this embodiment the total dispersion is substantially linear (total dispersion slope is substantially constant) over the pre-selected wavelength range.
In another embodiment of this aspect of the invention, the center region includes at least seven structural elements arranged as six structural elements surrounding a centrally positioned structural element. The centrally positioned structural element has a refractive index profile which guides light and so it properly denoted a core. At least three of the surrounding structural elements are cores.
In another embodiment of this aspect of the invention, the center region contains at least seven structural elements of substantially equal diameter arranged as six structural elements surrounding a centrally positioned structural element configured to be a core. At least three of the surrounding structural elements are configured to be cores. The centrally positioned core and the three surrounding cores each contain a dopant material that serves to increase the relative refractive index percent of respective portions of the respective cores. When the dopant material causes the refractive index of the core portion to increase, the value of the relative refractive index percent of the doped portion is positive, as can be seen from the definition of relative refractive index percent given below. The remaining three surrounding structural elements have a uniform refractive index. The cores having a uniform refractive index can be fabricated without use of a dopant material, although a dopant material can be used to uniformly raise or lower the refractive index of the core relative to that of the clad layer. In the case where a structural element of the center region of the multiple core optical waveguide fiber has a relative refractive index percent equal to or less than that of the cladding layer, the structural element does not function to confine light to the fiber and so is denoted a spacing element. The six surrounding structural elements can advantageously be arranged so that each surrounding core containing a dopant material over a core portion is neighbored by two surrounding cores of uniform refractive index. The cores containing a dopant material over a core portion preferably have their portions of increased relative refractive index percent positioned to include and be symmetrically distributed about their respective centerlines. Preferably, the centrally positioned core has a portion having a relative refractive index percent (xcex94%) of approximately 2.0%, the portion having a diameter of approximately 3 xcexcm. In the context of reference to the xcex94% and radius of the core or clad of an optical waveguide fiber, the term approximately generally means xc2x1/xe2x88x9210% of the nominal value stated. This 10% tolerance will be understood to pertain to all relative refractive index percent and radius values stated throughout the specification. Also preferably, the surrounding cores containing a dopant material over a core portion each have a portion of relative refractive index percent of approximately 1.0%, the portion having a diameter of approximately 6.4 xcexcm. The surrounding cores having a uniform relative refractive index percent over the core preferably have a relative refractive index percent of approximately 0.7%. Preferably in this embodiment each of the seven cores has an outside diameter of approximately 12 xcexcm. As is described in more detail below, any of the core portions having a non-zero relative refractive index percent can be characterized by a particular refractive index profile shape, which is represented in two dimensions as the curve of relative refractive index percent versus radius. For example, the centrally positioned core preferably has a refractive index profile which is a step although this profile can be an xcex1-profile, as defined below, with the xcex1 parameter equal to approximately 2.
An alternative embodiment of a multiple core optical waveguide fiber including seven structural elements exhibits a configuration similar to the configuration described immediately above. Preferably, the centrally positioned structural element is a core having a portion having a relative refractive index percent of approximately 2.0%, the portion having a diameter of approximately 3.5 xcexcm. In this embodiment, the refractive index profile of the centrally positioned core is an a-profile having an a of approximately 2, although the index profile can also be a step. Also preferably, three of the surrounding structural elements are cores containing a dopant material over a core portion each have a relative refractive index percent of approximately 0.35% over the portion, the portion having a diameter of approximately 10.9 xcexcm. The remaining three surrounding structural elements are spacing elements, that is, they are of uniform relative refractive index percent and are essentially silica and so have a uniform xcex94% of zero. Preferably in this embodiment each of the four cores and three spacing elements has an outside diameter of approximately 13 xcexcm.
Another aspect of the present invention is a method of making a multiple core optical waveguide fiber. A plurality of core or spacing element preforms are fabricated, each of the respective preforms having a refractive index profile and at least two of the core preforms having respective refractive index profiles that are different from each other. The preforms are bundled to form a multiple core preform structure. A clad layer is deposited or positioned on the bundled preforms to form a draw preform, which is drawn into a multiple core optical waveguide fiber. The clad layer may take the form of a tube in which the cores are placed. The cores may be bundled before placement in the tube or the tube itself may serve as the bundling structure. The multiple core preform structure is configured so that light propagating in the drawn fiber couples between at least two of the multiple cores of the drawn fiber and total dispersion and total dispersion slope of the drawn fiber are negative over a pre-selected wavelength range. The amount of coupling depends upon the respective refractive index profiles and center to center spacing of the cores between which light couples. The factors that determine the amount of coupling serve to provide the desired properties of the multiple core waveguide fiber.
In an embodiment in accord with the method, at least one of the core preforms includes a dopant material to increase the relative refractive index percent of a portion of the core preform.
In a further embodiment in accord with the method, the plurality of core or spacing elements preforms includes at least seven preforms configured such that a centrally positioned preform is a core preform surrounded by six preforms. Preferably, the seven preforms have substantially equal diameter so that the surrounding preforms abut or nearly abut the centrally located preform as well as two neighboring preforms.
Another aspect of the present invention is an optical waveguide fiber link in which transmission fiber dispersion is compensated by a fiber in accord with the invention. The link includes at least a first and a second length of optical waveguide fiber optically coupled to each other in series arrangement. The first fiber length can be taken to be the transmission fiber having a positive total dispersion and positive total dispersion slope over a pre-selected wavelength range. The second fiber length can be taken to be the compensating fiber having a negative total dispersion and negative total dispersion slope over the pre-selected wavelength range. Each of the first and second fibers is characterized by an end to end dispersion defined as the total dispersion multiplied by the fiber length. In accord with the convention usually used in the art, a fiber is said to have positive total dispersion if shorter wavelength light travels at a higher speed in the fiber than does longer wavelength light. A fiber having negative total dispersion is defined conversely.
The second fiber length, the fiber length having negative total dispersion and negative total dispersion slope, is a multiple core fiber in accord with the invention. Light propagating in the multiple core fiber couples between at least two of the cores. The total dispersion and total dispersion slope of the first and second fibers and their lengths are selected such that the sum of the respective end to end dispersions of the first and second fiber is less than a pre-selected value over the pre-selected wavelength range.
A preferred pre-selected operating wavelength range is from 1490 nm to 1650 nm and more preferably from 1500 nm to 1600 nm. A yet more preferred pre-selected wavelength range is 1525 nm to 1565 nm.
The sum of the respective end to end dispersions over the pre-selected operating wavelength range is preferably less than or equal to 25 ps per nano-meter of source spectral width and more preferably less than or equal to 15 ps/nm. Preferably, the sum of the respective end to end dispersions over a pre-selected operating wavelength range from about 1525 nm to 1565 nm is less than approximately 12 ps/nm. The preferred sums of end to end dispersion can be reached by matching the total dispersion and total dispersion slope of the respective first and second fibers. As an alternative, a third fiber can be introduced into the link to compensate residual dispersion. The third fiber can have essentially any combination of respective signs of total dispersion and total dispersion slope to compensate residual dispersion. For example, a step index single mode optical waveguide fiber, such as Corning SMF-28(trademark) can be used as the third fiber. In the case where the third fiber has a positive total dispersion and positive total dispersion slope, the magnitudes thereof are different from those of the fiber having total dispersion and total dispersion slope of positive sign.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operations of the invention.